1. Field of the Invention
The invention relates generally to the field of drilling wellbores through subsurface Earth formations. More specifically, the invention relates to methods and systems for creating fractures in Earth formations during the process of drilling such formations.
2. Background Art
Wellbores are drilled through subsurface Earth formations, for among other purposes to extract useful fluids such as petroleum. A wellbore provides an hydraulically conductive path from a permeable, fluid-bearing subsurface formation to the Earth's surface, such that fluids present in the pore spaces of such formations may be moved to the surface through the wellbore.
Certain types of subsurface formations may be susceptible to reduction in their permeability near the wellbore as a result of the drilling of the wellbore. Such reduction in permeability is known as “skin damage” and may result in the particular wellbore producing substantially less fluid and/or at substantially lower flow rates than would have been predicted based on the fluid flow properties of the particular formation. Other subsurface formations have relatively low natural permeability. For circumstances as explained above, among others it is known in the art to hydraulically fracture such subsurface formations. Hydraulic fracturing typically includes pumping fluid into the formation at rates sufficient to create pressures that exceed the pressure at which the formation will break or rupture. Once a fracture is initiated, pumping of fluid may continue for a selected period of time so that the fracture will extend a selected lateral distance from the wellbore. When the fluid pumping stops, however, the weigh of the formations above the fractured formation would cause the fracture to close. Fracture fluids known in the art therefore include a suspension of various solid particles called “proppant” that resist crushing and consequent closing of a fracture after pumping stops. The effect of the fracture is to increase the effective radius of the wellbore. As is known in the art, the rate at which fluid flows from a subsurface reservoir is related to, among other factors, the ratio of the wellbore radius with respect to the subsurface reservoir radius.
The above fracturing procedures are typically performed after drilling a wellbore is completes and a protective pipe or casing is placed in the wellbore. There may be substantial savings of time and cost if fracturing were performed during the drilling of a wellbore, so that particular subsurface formations could be evaluated earlier in the well construction process.